Appendices. Appendix 1, Description of the
turn involved.
Appendix 2, Glossary of terms
There are many subtly different types of parallel turn in alpine skiing. This reports aims to study one particular type of turn: The open parallel turn with up initiation across the fall line. A parallel turn can be split into 4 significant stages
The preparation stage is where the timing and positioning of the turn will be made, the skis are in the parallel position, and the knees flexed.
Initiation, is effectively lifting the skis off the snow with a slight jump, the up-initiation, or extension. A rotary motion of the skis and slight banking of the body leads the turn into the next phase.
The steering phase; the actual turning of the skis, in this case, they are rotated about the tip, preceded by a transfer of weight from the downhill ski, to the uphill ski. This is where the change of direction takes place.
The end phase, The completion of the turn. This allows the skier to either continue turning uphill so as to stop, to continue traversing such as in acyclic turns, or to prepare another turn, such as in cyclic turns like a wedeln or short swing.
The preparative and end phases, are very much a decision process made by the skier, and so are not relevant biomechanically. The areas of concern in this report are the phases of initiation and steering.
A simple description of how the turn is performed can be seen in appendix 1.
All forms of parallel turn rely on some form of unweighting of the skis
to enable them to turn. There are two recognised forms of unweighting
involved in parallel turning, up-unweighting and down-unweighting.
In this case we will be examining a turn that uses up-unweighting (Figure
1), sometimes referred to as flexion-extension-flexion. Within the
number of turns using this technique of initiation, we will look at those
that use an unweighting motion initiated by the outside ski only
(upstem), as this allows a more obvious demonstration of how the forces
act on each edge of the ski, and thus on the anatomy
of the skier. The data used comes from a study performed by Muller in 1992.
This sample was examined on a steep(24°) slope of well packed snow.
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The initiation phase is linear relative to the ground, but towards the end of this phase, the skis are starting to follow an angular motion, relative to the skier, as they are rotated into the start of the steering phase. During the traverse preceding the initiation of the turn, the weight of the skier should be distributed between the downhill ski and uphill ski by a ratio of 3:1, with a forward leaning stance on the downhill ski spreading the load between the front and back of the ski with about even pressure on the snow. This is due to the fact that although more force is being applied to the ski in front of the boot, the |
| of the ski has a larger contact area with the snow than the rear, and
so the pressure is spread evenly.The up-unweighting is carried out by exerting
a force on the snow from the downhill ski, inside edge.
By standing up from the flexed position in the way, the force the skier
exerts on the snow is momentarily changed (Figure 2).
We can see that in the traverse, the skier is exerting 100% of his body weight through the inside edge of the outside ski, and with the force applied to make the upward motion this force is more than 120% of the skier's body weight. At the uppermost point of this motion, the force on the snow is reduced to about 50% of the skier's body weight, and this is applied to both skis. This is the point at which the weight is transferred from the outside to the inside ski. The inside ski will become the outside ski after the turn is completed. |
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The steering phase of this turn follows an idealised arc round a radius.
However, since it is impossible to perfectly carve
a turn, we can only apply Newton's laws in theory. The actual force
systems involved are too complex to be discussed here.
The forces involved in this steering motion are angular. The skier, by following an arc, generates a centripetal force to maintain a turning motion, (Figure 3 top) and stop himself flying off at a tangent (Figure 3 lower). This is done by edging the outside ski, inside edge, and transmitting a force through it. This action can be seen again in Figure 2, towards the end of the turn. In this figure, the force line to look at is that of the uphill ski inside edge, as by this time, the weight has been transferred. As can be seen, the force applied to this ski increases throughout the turn. This is due to the fact that the skier gains momentum throughout the turn as gravity accelerates him down the fall line. The force acting against this centrifugal force is applied through the skier banking his body, much the same was a motorcyclist banks his bike during a turn (Figure 4). The degree of angulation depends on the degree of force that must be overcome. The force exerted on the snow by the skier during this phase can be
as high as 160-175% of his body weight. Typically, and in good conditions,
this force is split between the uphill and downhill ski by 25% and 75%
respectively. The degree of force applied to the outside ski, inside
edge is again relative to the radius of the turn and
the angular velocity of the skier, as is the degree of distribution of
the forces between the uphill and downhill ski.
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In Muller’s study, the muscle activity was measured using an electromyograph.
this measured the activity at the tibialis anterior, peroneus longus, gastrocnemius,
vastus lateralis, vastus medialis, rectus femoris, gluteus maximus and
erector spinae. The results of the EMG for this turn technique can
be seen in Figure 5.
This EMG shows that during the upstemming process, where the centre of gravity is moved to change the edge pressure from one ski to the other, causes the tibialis anterior and the peroneus longus to work harder. These are the muscles that would be involved in the up-initiation, or extension. Any turn requiring the upstemming motion would require such activity from these muscles. There was, perhaps surprisingly, little effort required from the thigh muscles. The hip and knee extensors contributed little during the actual steering phase, where the angle needs to be held to combat the centripetal force. The only muscle that was worked continually was the tibialis anterior. |
By understanding the biomechanics behind the execution of this particular
turn, it would aid many skiers in their performance. The technique
discussed herein, is performed mainly by beginners. Its exaggerated
motions become effective in all conditions, be it ice or packed snow, steep
or shallow gradients. Often advanced skiers resort to this manoeuvre
in difficult conditions or bad visibility. Discussion of the mechanics
of turns like this are an aid to instructors and coaches, and some of the
more advanced studies cover more techniques, in more conditions, and as
such are more comprehensive. This sport is one which lends itself
to biomechanical analysis, and so, is likely to benefit from it.
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Click on the picture below to see the full diagram.(Large Jpeg)
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Acyclic turns:- Turns performed with a traverse in between each one. Not a continuous swinging motion.
Angulation:- A body position where the knees and hips are pushed into the slope and the head and upper body lean out from the slope. It is used to maintain balance while edging the skis
Camber:- The arch built into a ski which means it curves upwards in the middle when lying flat on the ground. It is designed to distribute the skiers weight more evenly
Carve/Carving:- Turning with minimum skidding along a groove cut by the inside edges of the skis. Carved turns are due to the sidecut and camber of a ski. These can only made when there is adequate force to cause a reverse camber in the ski.
Centrifugal Force:- The force acting on a body, trying to throw it outwards, when it's rotating.
Centripetal force:- The force a body applies to prevent itself being thrown outward, combats centrifugal force.
Cyclic turns:- Turns such as Wedeln and short swinging, whose preparative phase is the end phase of the previous turn
Edge:- The metal strip running along the sides of a ski's running surface. Protects the ski and provides a bite into the snow when turning.
Edging:- tilting one or both skis onto its edge.
End phase:- The final stage of the turn where the centripetal force is stopped, and the skier continues in a straight line
Extension:- Stretching the hip, knee and ankle joints, See up-extension.
Fall-line:- The shortest and fastest line down a slope.
Flexion:- Bending of the hip, knee and ankle.
Initiation:- When turning, it is the point at which the skier begins to turn his skis out of the previously parallel travel.
Parallel turning:- Any turn where the skis are kept parallel from start to finish.
Reverse Camber:- When an edged ski is turning under load, its normal upward arch/camber, is reversed so that the ski bows downward in the centre and makes a carved turn.
Rotary motion/Rotation:- Rotating the body in the direction of the turn, in order to transfer a turning force to the skis.
Short swinging:- Tight radius turning where the end phase of one turn is the preparative phase of the next.
Side cut:- The waisted shape of the ski, narrower in the middle, and wider at tip and tail. This allows the ski to take on a reverse camber under load.
Traversing:- Skiing across the slope, at an angle to the fall-line.
Unweighting:- the process by which a skier momentarily reduces the force he exerts on his skis by performing up-extension. It is this momentary lightness that enables the skis to be turned.
Up-extension:- Rising into a high stance by stretching the hip knee and ankle extensors.
Uphill turn:- turning away from the fall-line, decreasing the angle of descent.
Wedel/Wedeln:- Rapid rhythmic short swinging down the fall line.
Weight transfer:- Shifting the Centre of gravity from over one ski to
the other.
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The Hamlyn guide to Skiing: martin Heckleman 1886, Hamlyn
Analysis of the Biomechanical Characteristics of different swinging techniques: Erich Muller, Journal of Sports sciences 1994 vol12, 261-278
Figures 1,2 & 5 taken from Muller, 1994
Figure 4 taken from “How we Learned to ski” Ali Ross Video.
Figure 3 taken from K.Gamma 1992
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